To be this pissed off with my 4 year old

I wondered if anyone was actually going to mention consequences here. A PP suggested you and your son have a cuddle!! He needs to know that what he did was wrong. At nearly 5 years old he is old enough to realise that behaviour like this has consequences.

No, it isn’t.

it’s ok for 4 year olds to be on their own in a room while a grown up is pottering in the next room!

I would be angry. He knew it was wrong. 4 is too old to be doing that.

Well no it’s not every child, but one under 5 should generally be monitored. If they are able to do things like this they’re being left too long.
If an adult is asleep they might as well be out of the house.

"Not this morning, it's X's birthday so she gets to choose."

Or

"Of course. X's birthday presents are down there for her to open later. I know it's tempting but please leave them alone."

Could have been avoided. Not helpful now though. Does he have issues with impulse control generally?

Exactly. I don't believe some of these responses. Yes, what he did was a bit naughty and I'm not saying he shouldn't face any consequences, but I find the OP's hyperbolic language a bit disturbing. At the end of the day, nobody was hurt or harmed. It's not like he killed the family cat!

OP I disagree with most of the posters here, I think YANBU

I'd have gone nuclear, especially as you say he's closer to 5 than 4.

Sometimes my 4 year old gets downstairs before we do, it's normal, not a big deal, I also have a 2 year old so can be always in the same room as 4 year old and I'm sure you're the same.

The reality is you do just have to take a deep breath and move on, but I would be absolutely fuming. A 4 nearly 5 year old should know better.

To be so angry at a 4 yr old who did something silly while unsupervised that you can't even look at them and post on an internet forum for others to back up your rage is outlandish.

"Put it aside" if it’s a one-off? Give him a big hug?? Why? I certainly wouldn’t. At 4 he’s old enough to know that what he did was very wrong (unless he has severe SN) and he should be receiving consequences, not rewarded with a hug or have his naughtiness ignored.

I have a child the same age and am really confused by this. Were you asleep when he was downstairs? What about things like turning the oven on, or spilling a kettle of boiling water over himself?

I let my DC go upstairs alone through the day because it's relatively childproof, but downstairs whilst I'm asleep seems quite dangerous.

In terms of the presents, he's 4 and has immature impulse control. Explain why what he did was wrong then salvage the day for your daughter.

Exactly ! Honestly sometimes people just love to jump in and make people feel worse than they already do

At nearly 5 he's definitely old enough to know better.
Yes you can be annoyed with him.
I certainly wouldn't be rewarding him for such behaviour.

So what were the consequences? I have twin 4 year olds and they’d get an absolute bollocking and there would be serious repercussions

He's only four, you are overreacting. 'Can't look at him'? That is super dramatic and disproportionate. That's what you say about an adult or maybe a child who has done something truly heinous, not just messed about with a few presents. I also agree that he may well be picking up on your low mood and acting up because of that. Children are very sensitive and it is bad for them when their parents are in a bad place.

Agreed, both my kids (15 months apart) would never have done this. I've always left presents out for Christmas/birthdays and thankfully not had this issue. One has ASD too so less impulse control but he's never done this either. Id be gutted too

The OP hasn’t said she’s beaten the 4 year old or even shouted at him which I know is an unforgivable MN sin.
She has said that inside, she is fucking furious with him for deliberately trashing his sister’s presents. The OP is allowed to have feelings that aren’t all sunshine and rainbows even about her own child. She also doesn’t have to blame herself for other people’s actions, even if the other person is only 4 years old.

You think it's unusual fir a 4 year old to misbehave?

I'd be absolutely livid too, I'd expect them to want to look, maybe try and lift the paper off the edges ro have a quick look at one but I actually think a 4 year old should know better and I'd be really upset and angry. And yes at 4 I let mine downstairs on their own.

I think this is unusual for a 4 year old, he knew the presents were for his sister and the fact he actually destroyed some of them speaks to bigger issues than unwrapping someone else's presents. DS and my DNs at that age wouldn't have done that nor their friends and they're not angel children!

Or maybe she's depressed so her wee 4 yr old is also having a confusing time with mummy not being herself and deserves some grace here

I have 2 children, if they're up and about in the morning I go down with whoever is down. Equally if I knew the living room was full of wrapped presents I wouldn't expect a potentially jealous 4 yr old to be able to entirely ignore that. As an aside they aren't allowed to just take themselves down and watch TV either or my 7 yr old would be up at 4am watching Pokémon. It's not that hard to keep an eye on 2 children and I say that with a 4 yr old and a 7 yr old with additional needs. It sometimes can get stressed if 1 is kicking off about something but ultimately on a normal morning I'd have a decent awareness of where they both are and what they're up to.

Could what happened to the op happen in my house? Yes! But my first thought wouldn't be to 'not even look' at my 4 yr old in rage it would be that I dropped the ball

I agree.
I assume he doesn’t normally rip up colouring books so I would also assume that he did it deliberately out of jealousy.
The OP has said she is depressed and this would hit hard as arranging a nice day for her DD will have been harder than for all the perfect mums who are judging.

You are allowed to be furious with your kids when they do things like this.